Thin-section petrography shows that plutonic rock that makes up the main part of Cowles Mountain is medium-grained hypidiomorphic hornblende monzogranite. In situ field measurements of magnetic susceptibility for apparently fresh monzogranite outcrops yield values from 17 widely distributed sites that average 3027 x 10-5• Magnetic susceptibility measurements at several of the sites however exhibit great variation; for example at one site, values over an area of ~20 m2 range from -400 x 10-5 up to ~5000 x 10-5• Thin section examination of two samples collected three meters apart with high and low readings, respectively, demonstrate that the great variation in magnetic susceptibility is independent of modal abundance of opaque oxides in the rock. X-ray diffraction analysis of opaque oxides collected with a hand magnet from these two rocks reveal however that the sample with the high magnetic susceptibility reading contains nearly pure magnetite, while the low reading sample contains hematite in addition to magnetite. The difference in opaque oxide mineralogy for these two samples is not detectable in thin section analysis by transmitted light. Results from this study suggest that significant variation in magnetic susceptibility from apparently fresh outcrops may be due to subtle effects of weathering and oxidation of magnetite. Magnetic susceptibility data from weathered outcrops of monzogranite on Cowles Mountain and from locally derived coarse sand on steep slopes of the mountain yield lower average values of 1873 x 10-5 and 638 x 10-5, respectively, which is consistent with progressive lowering of magnetic susceptibility as a result of oxidation of magnetite.